The size of each pixel of solid-state image sensors being used in digital video cameras and digital still cameras has been downsized year by year as the number of pixels of such a camera has been increased. As a result, the area of the photoelectric conversion section of the camera has been reduced, and the aspect ratio (depth/width) of the pixel structure has been raised. As the area of the photoelectric conversion section is reduced, there arises a problem of reduction in light receiving sensitivity. As the aspect ratio of the pixel structure is raised, there arise concerns that the F-value of the on-chip lens rises to reduce the focusing efficiency and produce a problem of crosstalk of light leaking to adjacent pixels.
To date, solid-state image sensors having an optical waveguide as disclosed in Japanese Patent Application Laid-Open No. H06-224398 have been proposed as means for solving the above-identified problems. A known solid-state image sensor having an optical waveguide will be described by referring to the accompanying drawings. The solid-state image sensor includes a plurality of pixel units 400 arranged in the form of a matrix. FIG. 15A schematically illustrates a cross-sectional view of a known pixel unit 400. The pixel unit 400 includes a silicon substrate 1101, a photoelectric conversion section 1102 arranged in the silicon substrate 1101 and a clad section 1103 formed of a transparent material on the silicon substrate 1101. A wiring section 1104 is formed in the inside of the clad section 1103 at a predetermined position on the silicon substrate 1101. Additionally, a core section 1105 is embedded in the inside of the clad section 1103 on the photoelectric conversion section 1102 to form an optical waveguide 1106. A refractive index of the core section 1105 is higher than that of the clad section 1103. A tapered light incident section 1108 is arranged on the optical waveguide 1106 so as to gradually increase its width toward the light incident surface 1107 and a color filter 1111 is formed in the inside of the light incident section 1108. With the solid-state image sensor having such a configuration, light entering it from the light incident surface 1107 of the light incident section 1108 can be converted into the guided mode that the optical waveguide 1106 possesses and converged to the core section 1105 for propagation. As a result, light can be guided efficiently to the silicon substrate surface 1110, while being protected against the crosstalk that arises above the silicon substrate 1101.
Japanese Patent Application Laid-Open No. 2008-41847 proposes a method of suppressing the spread of light in the transversal direction in the inside of a silicon substrate by means of an arrangement as described below. Namely, the spread of light in the transversal direction in the inside of a silicon substrate 1101 is suppressed by arranging a halftone light-shielding film on a silicon substrate 1110 to thereby invert the phase of the skirt section of incident light and suppress the spread of the electric field intensity distribution on the silicon substrate surface 1110.